<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>07-预解析案例</title>
</head>
<body>
    <script>
        // 预解析案例
        // 案例1

        function cook(){
            console.log(age);
        }
        cook();
        var age = 10;  // undefined 位置的不同，输出不同

        // 案例2
        var num = 10;
        function fn(){
            console.log(num);   //undefined
            var num = 20;
            console.log(num);   // 20
        }
        fn();
         /* 
            预解析后
            var num;
            function fn(){
                var num;
                console.log(num);
                num = 20;
                console.log(num)
            }
            num =10;
            fn();
            
        */
        
        // 案例3
        var a = 18;
        f1();
        function f1(){
            var b = 9;
            console.log(a); //undefined  
            console.log(b); //9
            var a = '123';
        } 
       
        /* 相当于以下代码 */
        var a;
        function f1(){
            var b;
            var a;
            b = 9;
            console.log(a);
            console.log(b);
            a = '123';
        }
        a = 18;
        f1();
    </script>
</body>
</html>